Interesting article in Model Boats web site which mentions joggling, however I'm still none the wiser.
http://www.modelboats.co.uk/forums/postings.asp?th=119534
and in addition an interesting article on decking in the Titanic Research and Modeling Association web site.
http://titanic-model.com/db/db-02/bruce-1-db-02.html
after some more research I found this comment by Ian Gardner
"I find, when joggling planks into the waterway or king plank, it is useful, if you have the facilities, to grind a chisel the width of the joggling to chop out the recess in the waterway. This is usually one third the width of the plank being used if memory serves. The chisel can be made from a an old needle file and hardened after grinding."
and pictures of joggled joints it appears its cutting into the waterway or king plank.
In a way you can see the reason its called joggling as it gives the same profile as in metal joggling eg "Z"